This is the basic problem with any investment in fossil resources. The assets will be used regardless of who owns it. So we need to think more upstream on this problem. We need to shut down further investment into new fossil assets.
The unintended consequence of a government buying up distressed assets is that it provides basic price support for those assets and thereby induces continued investment in new assets.
Keep in mind the dysfunctional economics of US shale. Generally, investors have lost money. But so long as there are buyers for wells, E&P outfits will drill those wells. It is irrelevant if those well ultimately do not produce enough economic value to justify the investment. E&P companies are not selling oil; they are selling wells. As long as there is a buyer for wells, drillers will keep drilling.
So for me, the enormous risk of nationalizing oil companies is that the government become a willing and capable buyer of wells and other assets. If the government buys wells, just to shut them down. This only provides price support to oil via reduction in supply. Thus, other investors will come along to buy up the additional wells that drillers drill. So ultimately there is no real reduction in the amount of oil consumed, but drillers get compensated for drilling wells that the government shuts down. It is more important to shut down financing to the drillers, than to close in wells.